This invention relates to an intensely colored transparent and/or translucent paper, more specifically an intensely colored tracing paper or an intensely colored transparentized paper.
It is already known that tracing papers are made from highly beaten cellulose fibers. Such papers are transparent and are used in technical drawing or by architects. Such papers are greyish in color.
It is also known that transparent papers are obtained by transparentizing opaque papers by means of chemical compositions based on a non-volatile liquid or wax. These papers are commonly called transparentized papers.
It is also known that transparent or translucent papers, colored or not, are obtained from initially opaque paper by means of high pressure hot calendering.
This kind of paper, called xe2x80x9cglassinexe2x80x9d is easily recognizable by the extreme smoothness of its surface. This smoothness is the direct result of the heavy pressure to which it is subjected in the calender to make it transparent. The Bekk smoothness value usually exceeds 2,000 seconds for paper of this type.
The technique used to make the basic paper does not include beating the fibers to a high degree before the sheets are formed. When such a paper is colored, the coloring agents are not easily fixed on the fibers and it is difficult to obtain paper with an intense color.
It is also known that transparent or translucent papers, colored or not, are obtained from an initially opaque paper by the sulfurisation process.
This kind of paper, called xe2x80x9cparchmentxe2x80x9d or xe2x80x9cvegetal parchmentxe2x80x9d is made transparent by chemical treatment. The opaque basic paper is immersed in a bath of sulfuric acid which dissolves part of the cellulose of the fibers. This bath is followed by repeated rinsing in water during which the cellulose recrystallizes while the acid in excess is eliminated. This does not facilitate fixation of coloring agents and it is therefore difficult to obtain intensely colored papers.
It is also known that tracing papers exist which are slightly bluish in color and are used by architects along with xe2x80x9cdiazoxe2x80x9d paper to reproduce plans. Likewise, it is known that tracing papers exist which are slightly pinkish and are used for technical drawing.
DE-C-603 554 describes a process for making printing paper, board for printing or other board by assembling damp sheets of unfinished paper containing dyes or fillers with layers of ordinary paper. This process is such that fillers or dyes are added in quantities equivalent to once or twice the dry weight of the fibers and the layers of paper are thus stratified. This document therefore concerns the manufacture of paper or board which comprise several layers of fibers. In no way does this document concern the manufacture of a colored tracing paper.
EP-A-0 097 371 relates to a process for making paper or other similar products by putting fibrous matter in suspension in a pulper, grinding the pulp to shorten, fibrillate and hydrate the fibers, sometimes mixing the stock of fibers obtained with additives, coloring agents and/or binders, and draining it on a wire, a process wherein 10% to 35% of the weight of cellulose is replaced by reinforcing agents. These reinforcing agents may be reinforcing agents derived from maize, rice and wheat. Consequently, this document certainly does not relate to the manufacture of tracing paper. It describes only the manufacture of a paper, which may be colored in the stock, and with additives derived from corn, rice, etc.
US-A-2 128 739 relates to a laminated paper made from two sheets of paper between which a wax film is laid. A laminated sheet is obtained which is greaseproof and waterproof. A colored or dyed product may be added to the stock.
This document describes the coloring of a paper during the papermaking process by adding a coloring agent. It is clearly stated that the gloss and opacity of the paper are maintained. The coloration of the paper is controlled by measuring the value of brightness L, the value a which is the color difference between red and green and the value b, which is the color difference between yellow and blue. This document in no way describes the manufacture of a colored tracing paper. It describes only the way the coloring of a paper can be adjusted by measuring L, a and b.
DE-C-250 533 relates to a process for coloring paper in the stock. The coloring solution is piped into the headbox.
JP-A-49 125 614 describes the reduction of the wood stock by means of a hydrosulfite and soda reducing agent. The pulp is mixed with a water-soluble coloring agent. It is therefore not a process for making tracing paper.
This invention aims to provide a translucent and/or transparent paper comprising highly refined fibers beaten to a very high Schxc3x6pper degree, of 50 to 80 or at least higher than 80, or a paper transparentized by means of a chemical compound, which has an intense color and Bekk smoothness below 2,000 seconds, preferably below 30 seconds, the cellulose of which is 100% in the form of fibers and which can be used for printing and/or writing.
This invention also aims to provide a tracing paper containing a high percentage of coloring agent.
This invention also aims to provide a highly colored tracing paper with an even look through.
The invention also aims to provide a process for making highly colored tracing paper or a highly colored transparentized paper.
Men skilled in the art of papermaking know that there are several methods for coloring a non-transparent paper.
The first method is to print the paper on both sides. The inventors carried out many printing trials on the front and back of translucent and/or transparent paper sheets, especially tracing paper obtained from highly refined fibers. However, when both sides of a sheet of tracing paper are printed, whether by flexography, photogravure or offset, transparency is greatly altered and printability deteriorates.
A second method for coloring paper is to coat the front or back of the sheet, this coating being done with a composition containing coloring agents, fillers and a binder. The inventors therefore coated a translucent and/or transparent paper with a colored composition. However, the higher the percentage of coloring agent, the greater the loss of transparency.
A third method for coloring paper is to introduce a coloring agent in the aqueous solution of cellulose fibers. This technique can be used only if the coloring agent is introduced before the circuit in which the paper pulp is mixed with water and the other traditional ingredients in the paper. The coloring agent must stay in contact with the cellulose fibers long enough for the coloring agent to be fixed in the fibers. Men skilled in the art of papermaking know that if the amount of coloring agent is increased, only part of the coloring agent fixes in the fibers, because once the fixation sites on the fibers are occupied by the coloring agent, no further coloring agent can be fixed. Therefore men skilled in the art of papermaking think that it is impossible to fix more than about 5% by weight of liquid coloring agent in relation to the finished paper. Moreover, the more colorant we introduce, the more is discharged in the underwire waters, which leads to pollution of the machine circuits and the environment.
A fourth method is to color the tracing paper after the drying cylinders, in the size press. However such a process does not give the paper an intense tint.
Consequently, the inventors had to find ways to:
obtain a transparent and/or translucent colored paper which has a pronounced or intense tint;
obtain a transparent and/or translucent colored paper which stays transparent even though it has an intense color;
obtain a transparent and/or translucent colored paper which has a good look through, i.e. an even look through;
obtain a transparent and/or translucent colored paper which has a Bekk smoothness below 2,000 seconds and preferably below 30 seconds;
obtain a transparent and/or translucent colored paper in which the cellulose fibers are 100% in the form of fibers or fibrils;
obtain a transparent and/or translucent colored paper which has printability as good as that of uncolored tracing paper;
obtain a transparent and/or translucent colored paper by a manufacturing process which causes little pollution in the machine circuits;
obtain a transparent and/or translucent colored paper by a manufacturing process which only slightly colors the underwire waters discharged by the papermaking machine.
Technically, all these problems seemed insoluble by men skilled in the art of papermaking, since increasing the color of the paper should have sharply reduced the transparency of the paper and increased pollution. Moreover, professional papermakers thought that increasing the amount of colorant above about 5% would not intensify the hue of the paper since the surplus colorant could not be fixed in the cellulose fibers.
But, the inventors, in opposition to the preconceived ideas of men skilled in the art of papermaking and after many tests, managed to obtain a transparent and/or translucent paper, that is, a paper comprising highly refined fibers (tracing paper) or a paper transparentized by a chemical compound, with an even look through and an intense hue. To this purpose, the inventors made a transparent and/or translucent paper comprising highly refined fibers with a Schxc3x6pper degree of 50 to 80 or at least higher than 80 or obtained by transparentization, with a Bekk smoothness below 2,000 seconds, preferably below 30 seconds, wherein the cellulose is 100% in the form of fibers and/or fibrils, and introducing a coloring agent in the stock. This paper has a defined intense or dark color such that the color intensity c (or chroma) and the brightness L measured on an instrument such as a Hunterlab or Datacolor instrument are as follows:
either the ratio of c to L is higher than 50%, preferably higher than 60% and even more preferably higher than 70%,
or L is lower than or equal to 60, preferably lower than or equal to 50,
or the ratio of c to L is higher than 50%, preferably higher than 60% and even more preferably higher than 70% and L is lower than or equal to 60, preferably lower than or equal to 50.
The inventors obtained such a paper, in an unexpected way, by refining the paper pulp to a Schxc3x6pper degree of 50 to 80 or at least above 80, and introducing the coloring agent just before the headbox, that is well after the pulper, in opposition to what has always been done in the traditional manner to obtain an intense color. The paper comprises from 5% to 10% in weight of liquid dye rapported to the weight of finished paper.
Without wishing to be bound by theory, the inventors think that because the fibers are highly refined, that is that they comprise many fibrils, the number of fixation sites for the coloring agent is increased and that, compared with the same weight of paper obtained with lightly beaten fibers, the quantity of colorant fixed can be increased up to four times.
Moreover, because the fibers are highly refined, the coloring agent can be introduced in the headbox because the contact time between the fibers and the coloring agent does not have to be very long since the fibers have many sites for fixing the coloring agent owing to the presence of numerous fibrils.
Moreover, when making transparent and/or translucent paper, the temperature of the fibrous suspension is maintained between about 80xc2x0 and 100xc2x0 C. to permit better drainage of the paper sheet. But these relatively high temperatures are highly conducive to the fixation of colorants in the fibers.
When making a paper transparentized by a chemical compound, the chemical phenomena differ from those mentioned above, and lead in an unexpected way to an intensely colored transparentized paper.
The inventors prepared an intensely colored transparentized paper in an unexpected way by making an aqueous suspension of fibers and a water-soluble coloring agent, by depositing this suspension on the wire of a papermaking machine, by removing the water by gravity, then by drying the sheet between two roller dryers, and by depositing on the sheet thus dried a transparentizing chemical compound and finally drying the sheet.
Thus, the introduction of the coloring agent in the headbox avoids polluting the part of the machine between the pulper and the headbox. As the coloring agent is held by the fibrils, the underwire water contains very little coloring agent and therefore pollution is reduced.
Besides, as the finished paper is transparent and/or translucent, each particle of coloring agent plays a visible role. The colorant in the mass of the paper is visible. On the contrary, for non transparent papers, only the colorant on the surface of the paper is visible and therefore, to obtain an intense hue, much more colorant must be introduced.
The invention also concerns a transparent and/or translucent paper containing a colorant in the stock, introduced in the headbox, on which a colorant in an aqueous medium has been deposited in the size press, to increase the intensity of the hue when the same colorant is used and to obtain textured effects when a different colorant is used.
The invention also concerns a transparent and/or translucent paper containing a fluorescent coloring agent, added to the stock or in the size press, and perhaps one or several other water-soluble coloring agents.